Thin film resistor

ABSTRACT

A RESISTIVE FILM IS DEPOSITED IN A GROOVE FORMED IN AN APPROPRIATE SUBSTRATE AND A PAIR OF TERMINAL PORTIONS FORMED ON THIS SUBSTRATE ARE CONNECTED TO RESPECTIVE OPPOSITE ENDS OF THE FILM AND ARE ESPECIALLY ADAPTED TO INTERCONNECT THE RESISTOR IN PRINTED OR ETCHED CIRCUITS. A PROTECTIVE MATERIAL CAN BE PROVIDED OVER THE FILM TO PREVENT CONTAMINATION. THE VALVE OF RESISTANCE IS ADJUSTABLE BY GRINDING OR OTHERWISE REMOVING A PORTION OF THE FILM AT THE UPPER EDGES OF THE GROOVE, UNTIL THE DESIRED VALUE IS ACHIEVED.

Jan. l2, 1971 B. soLow 3,555,485

THIN FILM RES ISTOR Filed March 27. 1969 V/ /bL K///// 22 IO 22 lO INVENTOR.

BENJAMIN SOLGVV United States Patent O 3,555,485 THIN FILM RESISTOR Benjamin Solow, North Hollywood, Calif., assgnor to Angstrohm Precision, Inc., Van Nuys, Calif., a corporation of Delaware Continuation-impart of application Ser. No. 678,330, Oct. 26, 1967. This application Mar. 27, 1969, Ser. No. 810,995

Int. Cl. H01c 1/02 U.S. Cl. 338-252 6 Claims ABSTRACT OF THE DISCLOSURE A resistive film is deposited in a groove formed in an appropriate substrate and a pair f terminal portions formed on this substrate are connected to respective opposite ends of the film and are especially adapted to interconnect the resistor in printed or etched circuits. A protective material can be provided over the film to prevent contamination. The value of resistance is adjustable by grinding or otherwise removing a portion of the film at the upper edges of the groove, until the desired value is achieved.

RELATED APPLICATION This invention is a continuation-in-part of copending application Ser. No. 678,330, filed Oct. 26, l967, now Pat. No. 3,469,226 issued Sept. 23, 1969.

FIELD OF THE INVENTION This invention relates to electrical resistors and more particularly to thin lm resistors adjustable to predetermined values.

BACKGROUND OF THE INVENTION It is advantageous in many instances to have a resistor which can be adjusted or trimmed to a particular resistance value to suit operating requirements. For example, in an R-C network, it would often be desirable to trim the resistance to select a particular time constant. However, adjustable resistors of conventional design have been found wanting for one reason or another. For example, one known adjustable resistor of conventional design employs a zig-zag or sinuous resistive pattern formed on a flat substrate, with portions of the resistive pattern being manually fracturable to select particular resistance values. This type of component is adjustable only in discrete steps, and, in addition, requires considerable care to achieve the intended adjustment without chipping or breaking the device. Another known adjustable resistor is the potentiometer, which generally includes a wire wound or film resistive path and a contact slidably movable along this path. Such continuously adjustable resistors are, however, subject to the influence of external effects which can affect the resistance value. For example, the resistance determined by the adjustment of the potentiometer can be affected by vibration, which can cause the slidable contact to move, and is also susceptible to disturbance from dust or other contaminants which may affect the electrical connection between the slidable contact and the resistive path. In addition, continuously adjustable resistors are unnecessary and too expensive for many purposes. Further, such devices are not commensurate in size with modern microcircuits since the mechanical structure required in a potentiometer to provide adjustment limits the achievable miniaturization.

Various techniques are known for adjusting the value of certain conventional resistors during a manufacturing process, but such adjustment requires complex equipment 3,555,485 Patented Jan. 12, 1971 ice and cannot be practically carried out by a user of the SUMMARY OF THE INVENTION Briefly, a resistor according to the invention comprises an electrically insulating body having an elongated groove formed in a surface thereof, with a lm of resistive material disposed within the groove. A pair of terminals are located on the insulating body and in electrical contact with the resistive film, the terminals comprising metallized end portions having conductive areas especially adapted for mounting and interconnection in printed or etched circuits. The device can be encapsulated or coated with a suitable protective material to prevent contamination of the resistive film. To adjust the value of the resistance, for example, by grinding through a portion of the protective material covering the groove and abrading away a portion of the resistive film in the groove. ln practice, the device can be connected to an ohmmeter or other resistance measuring instrument while the resistive material is being removed, so that the adjustment can be stopped when the desired value of resistance is achieved. Alternatively, the resistor can be adjusted while it is in circuit to thereby trim a particular circuit parameter dependent upon the resistance value. Once the desired value is attained, a protective material is coated or otherwise applied to the surface of the film exposed by the adjustment operation to complete its encapsulation.

DESCRIPTION OF THE DRAWINGS The invention will be more fully understood from the following detailed description, taken in conjunction with the accompanying drawings in which:

FIG. 1 is a greatly enlarged pictorial view of one embodiment of a resistor according to the invention;

FIG. 2 is a greatly enlarged bottom view of the embodiment of FIG. 1;

FIG. 3 is a greatly enlarged sectional elevation view of the invention taken along line 3-3 of FIG. 1;

FIG. 4 is a greatly enlarged sectional elevation view of the invention after adjustment of resistance value;

FIG. 5 is a greatly enlarged sectional elevation view of an alternative embodiment of the invention; and

FIG. 6 is a greatly enlarged sectional elevation view of the embodiment of FIG. 5 after adjustment of resistance value.

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawing, there is shown, to a greatly exaggerated scale, one embodiment of a resistor constructed according to the invention, and which is especially constructed and adapted for interconnection in printed or etched circuits. A generally square body 10 of electrically insulating material, such as ceramic or glass, a central portion containing a V-shaped groove 12 formed therein and extending along a diagonal of body 10, with diagonal end portions 14 and 16 metallized to provide connection to a resistive path formed in groove 12. The central portion is raised or elevated with respect to end portions 14 and 16 to permit adjustment of the resistance value without affecting the device terminals and without interference by the terminals. A metallized coating 18 is applied on all four surfaces of diagonal end portions 14 and 16 and also extends onto a portion of 3 respective ends of groove 12 to provide electrical connection to the resistive path formed therein. As illustrated in FIG. 1, portions 18a of the coating 18 are visible at each end of groove 12 with resistive film 30 formed thereover in intimate contact therewith to provide good electrical contact between the resistive film and the device terminals. Typically, the metallized terminal areas are formed by dipping the corners of body in a platinumgold paste, then firing the paste to form a deposited metal film. The bottom surface of body 10 can be dipped in a solder bath to provide solder pads 20 and 22 on the bottom surfaces of the metal film, these pads providing a means of efficient device interconnection in circuit. Alternatively, a gold coating can be provided on the upper surfaces of end portions 14 and 16 for those applications where wire bonding to these surfaces is desired. A glaze coating can be applied over the resistive film and over the metallized lm to provide protection from moisture or other contamination, or a casing of insulating material can be molded around the device, as will be described.

This device is especially suited for incorporation into printed or etched circuits since the terminal pads 20 and 22 can be placed onto corresponding conductive paths of a utilization circuit and soldered thereto. As illustrated in FIG. 1, terminal pad 20 is soldered to conductive path 24 formed on circuit board 26, and terminal pad 22 is soldered to conductive path 28 formed on the circuit board. The novel resistor can therefore be incorporated in microminiature circuits in a manner compatible with the small size and planar configuration of such microcircuits.

A groove 12 of V-shaped cross-section is formed in the raised central portion of body 10, extending between end portions 14 and 16, the slope of the groove being selected to provide a predetermined degree of resistance change for a given removal of resistive film, as will be explained hereinafter. Although shown as being V-shaped, the groove need not be of this particular shape but can also be of arcuate or other cross-section.

The resistive path is composed of a film 30 of resistive material deposited onto the surface of groove 12, and onto the portions 18a of the conductive film 18 formed on the respective ends of the groove. The outermost edges of resistive film 30' can be substantially ush with the outermost edge of the raised portion of body 10, or the resistive film 30 can also be applied over adjacent surfaces 36 and 38 of the raised portion. A glaze or other protective coating can be applied over resistive film 30 and over metallized film 18 to protect the films from moisture or other contamination. The glaze is usually transparent and is not discernible in FIG. 1, and can be applied over the entire body 10, leaving terminal pads 20 and 22 exposed for device connection.

In order to adjust the resistance value of the device, the outermost edges of film 30 are removed, for example by abrasion, thereby increasing the resistance of the resistive path by reducing the cross-sectional area of the resistive path. This removal is easily accomplished by running, for example, a grinding wheel or ultrasonic grinder across the top surface of the device to remove a selected amount of resistive material. As illustrated in FIG. 4, edges 32 and 34 of film 30 have been abraded, along with the adjacent surface of corners 36 and 38, to selectively increase the resistance. The slope of the groove is chosen to provide an intended degree of resistance change for a given removal of substrate material. For example, a relatively shallow groove permits a rather large variation in resistance for a rather small removal of material, while a steep groove permits a relatively small change in resistance for a given removal of material. To expeditiously determine the resistance value, the device can be connected to an ohmmeter via the device terminals to measure the resistance as the resistive material is being removed. It is evident that only a thin edge of film 30 is exposed to the environment after the abrasion operation, and thus, substantially all of the resistive film remains protected from the environment by the glaze coating. The edges of film 30 exposed by the abraision can be coated with a glaze or other suitable material to fully protect the trimmed resistor.

An alternative embodiment of the invention employing a molded protective casing is illustrated in FIGS. 5 and 6. A casing 4.0 of epoxy or other suitable insulating and protective material is molded or otherwise formed around body 10 and over resistive film 30 and metallized end portions 14 and 16. The bottom face of the device containing conductive mounting pads 20 and 22 are left unencapsulated to allow device connection in a utilization circuit. As seen in FIG. 5, the upper edges of resistive film 30 is covered with a thickness of encapsulating material. If the resistor is of correct value for a particular use, the resistor can be utilized in this form, as can the glaze coated version hereinabove.

To adjust the resistive value of this embodiment of the invention, the outermost edges of film 30 are removed, for example, by running a grinding wheel or ultrasonic grinder across the top surface of the device to remove the epoxy coating 42 over the top surface of body 10 and to remove the surfaces 36 and 38 of the corners, thereby reducing the effective area of the resistive path and correspondingly increasing the resistance thereof. As seen in FIG. 6, a portion 44 of casing 40 remains in the groove after adjustment of the resistance value, and only thin edges 46 and 48 of film 30 are exposed to the environment after the abrasion operation, and these edges can be coated with a suitable material to complete the protective encasing of the trimmed resistor.

The casing 40 in this latter embodiment serves not only to protect the device from its environment but also functions as a support to allow grinding of the resistive film without chipping the ceramic body on which the resistive path is formed.

Fabrication of the resistor is accomplished using well known filmr deposition techniques, and its method of manufacture need not, therefore, be discussed in detail. In brief, conductive film 118 is sprayed and fired onto end portions 14 and 16 and onto ends of groove 12 via suitable masks, or dipped and fired, and the resistive film 30 can be evaporated or otherwise applied through a suitably configured mask onto the surfaces of groove 12. Alternatively, as mentioned hereinabove, resistive film 30 can be applied over the entire top surface of the device, that is over groove 12 and adjacent surfaces 36 and 318. The application -of this resistive film can be accomplished without masking by placing a group of resistors in side by side contact with one another and applying the resistive film for example by spraying or dipping. A protective glaze or coating is then applied over films 118 and 30, or an integrally molded casing can be formed around the device, as described.

As a typical example of a resistor embodying the invention, a film of 100 ohms per square is formed in a ceramic body having typical dimensions as viewed in FIG. 2 of 40 mils x 40 mils and a height as viewed in FIG. 3 of 25 mils. The initial resistance is ohms, and this resistance is variable to 400 ohms by removal of about 30 mils width of film 30. Thus, a variation in resistance of 5:1 is easily achieved by removal of only a relatively minute quantity of ceramic substrate and resistive material.

The invention is not to be limited by what has been particularly shown and described as various modifications and alternative implementations will now occur to those versed in the art. Although a single linear grove has been illustrated, it should be understood that a sinuous resistive path can be provided for higher resistance values. And, for example, although the illustrated embodiment is of square configuration, other shapes can also be employed. tAs an alternative, corners 36 and 38 of the device of FIG. l can be ground away to provide a generally rectangular body having rectangular terminal end portions as in the above-referenced copending application.

What is claimed is:

1. A resistor comprising:

a body of electrically insulating material having a raised central portion disposed between first and second end portions and a groove formed only in the outermost surface of said raised central portion;

first and second terminals formed on respective end portions of said body and including a conductive coating formed on respective end portions of said body and on a portion of respective ends of said groove and also formed on a porition of the surface of said body opposite to said outermost surface;

a film of resistive material formed in said groove to provide a resistive path, said film of resistive material being in intimate electrical contact with said conductive coatings; and

solder pads provided on the portions of said conductive coatings formed on the surface of said body opposite to said outermost surface.

2. The resistor according to claim 1 wherein said body is of generally square configuration and the groove formed in said raised central portion is disposed diagonally thereacross, and wherein said end portions are respective diagonally opposite corners of said body, said conductive coatings being formed on the surface of said respective opposite corners.

References Cited UNITED STATES PATENTS 599,352 2/1898 ONeill 388-308 1,767,715 6/1930 Stoekle 338-311X 1,889,379 11/1932 Ruben 338-308X 3,282,730 11/1966 Johnston 338-308X 3,388,301 6/1968 James 317-10'1(A)X 3,469,226 8/1969 Solow 338-252 ELLIOT GOLDBERG, Primary Examiner 

